Others say the sensors arenít worth a crap (pun intended), but very trailer I have owned and worked has worked when they are clean. You probably have toilet paper hung up, did the monitor panel say empty when you first bought the trailer?
2012 Copper Canyon 273FWRET being towed by a 1994 Ford F350 CC,LB,Dually diesel.
Airlift 5000 bags, Prodigy brake control, 5 gauges on the pillar.Used to tow a '97 Jayco 323RKS.
Now an RVIA registered tech. Retired from Law enforcement in 2008 after 25+ yrs.